Does a 2,200 Å hump observed in an artificial carbonaceous composite account for UV interstellar extinction?

Abstract

The 2,200 Å hump in the interstellar extinction curve has attracted much attention since its discovery by Stecher¹. Graphitic^2,3 and carbonaceous grains⁴ and MgO (ref. 5) have been considered as possible candidates. Sakata et al.⁴ have reported a hump near 2,200 Å in the absorption spectrum of an extract from the Murchison meteorite, a C2 carbonaceous chondrite. The hump was presumed to be due to hydrocarbons having conjugated double bonds. It is highly probable that the interstellar extinction is caused by carbonaceous materials containing such conjugated double bonds. In the present experiment, carbonaceous materials were synthesized by injecting a plasmic gas mixture of hydrogen and carbon into a vacuum chamber. Extinction of the material was measured over the range 1,900–7,000 Å. The extinction profile of the synthesized material clearly exhibits the 2,200 Å hump, the average value for the central wavelength being 2,170±30Å.